The invention relates to a method of electronic wiring or circuit board preparation, and more particularly for providing flexible-film or rigid-board-supported circuitry with a solderable metal overlay confined to the circuitry surfaces.
Printed copper circuits or contacts are conventionally used in the manufacture of electronic circuit and wiring boards with the copper elements either being bonded directly to the surface during the cure of the board material, e.g. rigid boards, or in the case of flexible circuits, attached to a non-conducting plastic substrate by an acrylic adhesive. A typical flexible substrate is "Kapton", a polyimide formed from pyromellitic dianhydride. Copper circuitry of printed wiring boards tends to oxidize causing poor solderability which interferes with effective electrical interconnection of boards and their components. To overcome this, it is desirable to overplate the copper surfaces with a more durable metal to provide corrosion resistance, surface integrity, solderability and bondability to wire or pressure contacts. Overplating may be accomplished by electroplating in certain printed wiring board processes, but this cannot be done effectively when plating must be accomplished after etching. In such cases electroless plating is used, resulting in greatly improved efficiency particularly in the treatment of small components.
Applicant has found that electroless plating with nickel-boron provides a relatively pure nickel deposit (approximately 99 percent nickel and one percent boron) over the surface of copper circuitry components that is easily solderable, more ductile than nickel-phosphorous compounds, and provides a metal overplate that does not diffuse into the copper components. However, in electroless overplating of copper contacts adhesively attached to "Kapton" boards using nickel-boron, the utility of the finished board was significantly reduced or destroyed by unwanted deposit of the overplate metal upon the board and adhesive surfaces such as to cause short circuits between the circuitry elements. To prevent such deposit metal overgrowth, experiments were conducted to formulate an electroless activating solution to minimize or eliminate such deposits. Further, a method was sought to alter the substrate and adhesive surface chemistry so as to passivate, or neutralize reactive sites on the plastic areas to prevent subsequent conductive metal deposit thereon. The method disclosed herein achieves these purposes.